Concern over the depletion and environmental impact of fossil fuels has stimulated strong interest in the development of alternative energy sources. Significant investments in areas such as batteries, fuel cells, hydrogen production and storage, biomass, wind power, algae, and solar energy have been made as society seeks to develop new ways of creating and storing energy in an economically-competitive and environmentally-benign fashion. The ultimate objective is to minimize society's reliance on fossil fuels and to avoid production of greenhouse gases.
A number of experts have concluded that to avoid the serious consequences of global warming, it is necessary to maintain CO2 at levels of 350 ppm or less. To meet this target, based on current projections of world energy usage, the world will need 17 TW of carbon-free energy by the year 2050 and 33 TW by the year 2100. The estimated contribution of various carbon-free sources toward the year 2050 goal are summarized below:
Projected EnergySource Supply (TW)Wind2-4Tidal 2Hydro   1.6Biofuels5-7Geothermal2-4Solar600Based on the expected supply of energy from the available carbon-free sources, solar energy is clearly the most viable solution for reducing greenhouse emissions and alleviating the effects of global climate change. (See J. Esch, “Keeping the Energy Debate Clean How Do We Supply the World's Energy Needs?”, IEEE Proc. 98(1), 39-41 (2010).)
Amorphous semiconductors are attractive materials for solar energy applications. Among the amorphous semiconductors, amorphous silicon is known to be a particularly promising solar energy material. Unlike crystalline silicon, amorphous silicon is a direct gap material that has strong absorption over much of the solar spectrum. The strong absorption means that high efficiency solar cells can be formed from thin layers of amorphous silicon. As a result, solar panels based on amorphous silicon (or chemically-modified or structurally-modified forms of amorphous silicon, including composite forms of amorphous silicon that include nanocrystalline phases) are lightweight, flexible, and readily adapted to field use in a variety of installation environments.
S. R. Ovshinsky has long recognized the advantages of amorphous silicon and related materials as the active layer of solar cells and has been instrumental through his inventions and developments in advancing automated and continuous manufacturing techniques for producing solar and photovoltaic devices based on amorphous semiconductors or combinations of amorphous semiconductors with nanocrystalline, microcrystalline, polycrystalline or single crystalline semiconductors. Representative discoveries of S. R. Ovshinsky in the field of amorphous semiconductors and photovoltaic materials include U.S. Pat. Nos. 4,400,409 (describing a continuous manufacturing process for making thin film photovoltaic films and devices); 4,410,588 (describing an apparatus for the continuous manufacturing of thin film photovoltaic solar cells); 4,438,723 (describing an apparatus having multiple deposition chambers for the continuous manufacturing of multilayer photovoltaic devices); 4,217,374 (describing suitability of amorphous silicon and related materials as the active material in several semiconducting devices); 4,226,898 (demonstration of solar cells having multiple layers, including n- and p-doped); 5,103,284 (deposition of nanocrystalline silicon and demonstration of advantages thereof); and 5,324,553 (microwave deposition of thin film photovoltaic materials).
Current efforts in thin film photovoltaic material manufacturing are directed at increasing the deposition rate without impairing photovoltaic efficiency and, in the case of silicon-containing materials, without exacerbating Staebler-Wronski degradation. Higher deposition rates lower the cost of thin film solar cells and can lead to a dramatic decrease in the unit cost of electricity obtained from solar energy. As the deposition rate increases, thin film photovoltaic materials become increasingly competitive with fossil fuels as a source of energy. Presently, PECVD (plasma-enhanced chemical vapor deposition) is the most cost-effective method for the commercial-scale manufacturing of amorphous silicon and related amorphous semiconductor photovoltaic materials. Current PECVD processes provide uniform coverage of large-area substrates with device quality photovoltaic material at deposition rates of ˜1-5 {acute over (Å)}/s. This deposition rate, however, is insufficient to achieve cost parity with fossil fuels.
In order to enhance the economic competitiveness of plasma deposition processes, it is desirable to increase the deposition rate. To effectively compete with fossil fuels, it is believed that deposition rates of 100 Å/s or higher are needed. The deposition rate of prevailing plasma deposition techniques is limited by the high concentration of intrinsic defects that develops in amorphous solar materials as the deposition rate is increased. The intrinsic defects include structural defects such as dangling bonds, strained bonds, unpassivated surface states, non-tetrahedral bonding distortions, coordinatively unsaturated atoms (e.g. two- or three-fold coordinated silicon or germanium). The structural defects create electronic states in the bandgap and near the band edges of amorphous semiconductors. The electronic states detract from solar conversion efficiency by (1) promoting nonradiative recombination processes that deplete the concentration of free carriers generated by absorbed sunlight and (2) reducing hole mobility. Intrinsic defects also contribute to degradation of the solar conversion efficiency of amorphous silicon and related materials through the Staebler-Wronski effect, an effect that leads to a 15-30% reduction in photovoltaic efficiency with use over time.
S. R. Ovshinsky has demonstrated that the concentration of intrinsic defects formed in a plasma-deposited material depends on the distribution of species present in the plasma. A plasma is a complex state of matter that includes ions, ion-radicals, neutral radicals and molecules in multiple energetic states. In particular, S. R. Ovshinsky has shown that certain charged species can be detrimental to the quality of as-deposited amorphous semiconductors under conditions in which they promote the creation of defects. Uncontrolled charged species tend to strike the deposition surface with high kinetic energy and damage a growing thin film material through bond cleavage. Bond cleavage creates dangling bonds and promotes the formation of locally strained coordination environments that may contribute to electronic defect states. S. R. Ovshinsky has shown that neutral species in a plasma, in contrast, frequently promote more uniform bonding and lead to lower defect concentrations in as-deposited material. S. R. Ovshinsky has ultimately showed that the proper balance of charged and neutral species is essential to maximizing deposition rate and minimizing defects. He has further demonstrated that the optimal identity, concentration, and charge of species in a plasma environment varies with plasma conditions and can be constructively influenced through chemical modification with agents such as fluorine.
To minimize the concentration of intrinsic defects, current plasma deposition processes are performed at low deposition rates. By slowing the deposition process, the intrinsic defects that form in the as-deposited product material have the opportunity to equilibrate to energetically-favored states that have more regular bonding configurations. As a result, the concentration of intrinsic defects is reduced. Unfortunately, the reduced deposition rate impairs the economic competitiveness of the process and prevents cost parity with fossil fuels.
In U.S. patent application Ser. Nos. 12/199,656; 12/209,699; and 12/429,637; S. R. Ovshinsky described techniques for minimizing the deleterious effect of uncontrolled charged plasma species on the defect concentration. The patent applications describe techniques for maximizing the presence of neutral species and controlling the presence and activity of charged species at the deposition surface through preferential formation of neutral species in the plasma activation process, magnetic confinement to regulate charged species, and/or separation of undesirable charged species to form a charge-controlled deposition medium. Through utilization of a charge-controlled deposition medium, high quality amorphous or other silicon-containing semiconductors can be formed at high deposition rates in a plasma deposition process.
Another strategy for increasing the deposition rate of plasma-based processes is to increase the plasma frequency. Conventional plasma deposition processes are typically completed at radiofrequencies (e.g. 13.56 MHz). As the plasma frequency is increased, the source gases used in plasma deposition are activated more efficiently, more completely, and to higher energy states. Plasma excitation at microwave frequencies (e.g. 2.45 GHz), for example, leads to higher dissociation rates of source gases, generates higher fluxes of ions and neutrals, and creates a higher proportion of plasma species (ions, neutrals) sufficiently energetic to participate in the deposition process. The high dissociation rates and higher excitation energies associated with microwave plasmas improve process efficiency by providing much higher utilization of source gases than radiofrequency plasmas. The high fluxes and energies of ions and neutrals produced by microwave plasmas lead to significantly higher thin film deposition rates than radiofrequency plasmas.
In addition to dissociation of a higher fraction of source gases, the high deposition rate accompanying microwave deposition of thin film precursors is also a consequence of the enhanced reactivity of deposition intermediates. Enhanced reactivity of deposition intermediates results from the higher energy of activation available from microwave excitation. Microwave excitation produces deposition intermediates with higher internal energy by activating deposition precursors to higher energy electronic and vibrational excited states. The higher internal energy makes the deposition intermediates less stable and more conducive to the structural rearrangements and reactions on the deposition surface needed to form a thin film material.
Although enhanced reactivity of deposition precursors is beneficial from the standpoint of deposition rate, it oftentimes leads to unintended side effects. A common problem in microwave deposition is the tendency of reactive deposition intermediates to form thin films away from the substrate. Thin film coatings, for example, may develop on the interior walls of the deposition chamber and may serve as a source of contamination for subsequent depositions.
Since the deposition chamber is normally operated under vacuum or with a controlled atmosphere, it has a limited volume and receives precursors, background gases, and energy from external sources. Materials are generally delivered by conduits through valves that pierce the boundaries of the chamber. Electrical energy (such as the bias between electrodes needed to initiate a plasma or the resistive dissipation used to heat a substrate) is typically supplied by wires that connect an external power source through the boundaries of the chamber to internal components. The formation of thin film coatings on the openings or actuators of internal valves, or on internal components such as electrodes or wires, may alter deposition conditions, impair the uniformity of deposition or prevent deposition altogether.
Unintended thin film coatings are particularly problematic when they form on the windows of a deposition chamber through which the electromagnetic energy used to activate a plasma from deposition precursors is transmitted. In microwave deposition, for example, the microwave generator is normally located remote from the deposition chamber. The generator produces microwaves and transmits them along a microwave waveguide to the deposition chamber or a downstream applicator, where the microwaves pass through a window to energize deposition intermediates or activate deposition precursors to form reactive species used to form a thin film material. To maximize the microwave energy coupled to the deposition intermediates or precursors, it is necessary to insure that the window is highly transparent to microwave frequencies. If the reactive species generated by the microwaves deposit the thin film material on the window and the thin film material absorbs microwaves, the transparency of the window decreases.
Decreased transparency of the window leads to two detrimental effects. First, any decrease in transparency leads to a reduction in the microwave energy coupled to the deposition intermediates or precursors. Reduced microwave coupling means that the deposition species are less dissociated, less energetic, less reactive, and as a result, the deposition rate decreases. Second, continued exposure of a microwave-absorptive thin film on the window to microwave radiation leads to localized heating of the thin film material that can cause thermal stresses and potentially catastrophic failure of the window.
The detrimental consequences of thin film coatings on microwave transmission windows do not arise if the coating is transparent to microwave radiation. Most dielectrics (including quartz, sapphire, diamond, boron carbide, SiO2, and Si3N4) are highly transparent to microwave radiation and may be formed safely at high deposition rates in a microwave plasma process. Coatings made from lower bandgap materials (including metals and most semiconductors), however, are much less transparent to microwave radiation and present much more serious concerns over safety and process consistency. Many desirable photovoltaic materials, including amorphous silicon and silicon-germanium, absorb microwave radiation and are difficult to manufacture in a microwave plasma process because the high reactivity conditions present in a microwave plasma promotes the formation of undesirable coatings on the windows used to transmit microwave radiation to the deposition environment. Accordingly, there is a need for a process that permits microwave deposition of semiconducting photovoltaic materials.